This invention relates to fiberoptic lasers and more particularly to control of the angular intensity distribution of light emission from a zero angular momentum radial fiber laser.
From the incandescent lamp to the modern solid state laser, the challenges associated with controlling the angular intensity distribution of light emission remain largely unresolved. While quasi-Lambertain sources form the basis for most modern displays, the lack of angular resolution presents a limitation in any future attempts to emulate light scattering events that have specific angular distributions, such as, e.g., in visual perception.
The directionality of light emitting devices is generally controlled by external mechanical and/or electronic beam steering optics. However, integrating beam manipulation capabilities directly within the light emitting structure can lead to more compact and functional light sources. On-chip control of beam directionality has been demonstrated in several laser configurations, including injection twin-stipe lasers [18], quasistadium microcavity lasers [20-21], and photonic crystal lasers [19]. The commonality in the physics used to modulate the Poynting vector distribution in references 18-21 relies on electrically varying the gain profile, which facilitates either beam steering [18], controlling the intensity ratio between multiple beams [20], or switching between one of two lasing modes [21]. The primary mechanism used to enable directional emission control in reference 21 relies on a tunable photonic crystal structure.
The emission characteristics of coherent light sources are determined by a delicate interplay between the gain medium and the cavity structure. Rotationally symmetric resonators are particularly interesting due to the possibility of omnidirectional emission in the direction perpendicular to the axis of symmetry. However, to date, all cylindrically symmetric sources rely on the excitation of whispering-gallery modes [1-6], characterized by essentially tangential wavevectors as shown in FIG. 1. The primarily azimuthally-directed arrows in FIG. 1 denote the tangential laser emission of the high angular momentum modes. The energy density plot corresponds to a high order whispering-gallery mode supported by the cylindrical structure. The polarization of this mode is indicated by the white dots overlaid on the energy density plot and black dots on the schematic. These modes are confined near the cavity boundary by total internal reflection and can only escape through diffraction losses or scattering from surface roughness. Consequently, the inherent drawbacks of these structures include limited control over the output coupling and diffraction-limited quality factors [17].
It is therefore an object of this invention to provide a cylindrical photonic bandgap (PBG) cavity that supports high-Q purely radial modes and allows full control over output coupling and the potential for scalability to small volumes without compromising the quality factor.